The present invention relates to an ohmic contact to a semiconductor body and a method for making same, and more particularly to such a contact for use with a compound semiconductor material such as GaAs or AlGaAs.
Various structures have been used to form an ohmic contact to a semiconductor body. One contact includes a conductive layer, such as a metal, disposed on a very heavily doped region of the semiconductor body. However, reproducible contact resistances are not obtained when the semiconductor body is composed of a Group III-V alloy, and further, the heavily doped region can degrade the surface morphology and crystal quality and therefore adversely effect the underlying active regions of the semiconductor. Another ohmic contact structure is formed by sintering a conductive layer, such as a Au:Ge alloy, with a III-V semiconductor body. However, the sintering step is expensive due to the required handling. Further, if the device also includes Schottky contact, a different metal and extra processing steps are normally required to form the Schottky contact. Another type of ohmic contact includes a lattice matched intermediate semiconductor layer between the conductive layer and the semiconductor body. However, the lattice matching is not always easily achieved and even then will be correct only for a body of one material. Yet another ohmic structure includes a conductive layer overlying two intermediate semiconductor layers. The lower intermediate semiconductor layer is epitaxially formed on the substrate and has a graded composition to be lattice matched to both the substrate and the upper semiconductor layer. However, control of the graduation is tedious and difficult to achieve in practice.
It is, therefore, desirable to have an ohmic contact and a method for making same that is consistent, does not affect active regions, is easy and inexpensive to make, can contact more than one type of semiconductor body, and allows the simultaneous formation of Schottky contacts.